I agree with the other responses but want to emphasize that it is not uncommon to end up on two different airlines when you don't have a non-stop flight. I don't let that stop me when I am pricing flights on Google Flights. However, there have been one or two occasions when I couldn't get a seat assignment on the second airline (not the one I bought the ticket from) until about 24 hours before departure. This doesn't always happen, but it does in some cases. I have always ended up in a seat that met my preference (regular Coach on the aisle), but it's a bit concerning, and it would be more problematic, psychologically, if I were not a solo traveler. Since it's the seat on the transatlantic flight that really matters, the issue can be avoided by buying the ticket from that airline. Sometimes, unfortunately, that will cost $25 - $50 extra (or conceivably more), so you have to decide whether the peace of mind that comes from knowing you'll be able to choose your seat right away is worth the extra money. The issues I've had have been with tickets purchased from United when the overseas flight was on a European airline (not all of them). Keep in mind that the super-low Basic Economy fare class (which goes under different names on different airlines) doesn't ever allow early seat selection.
Once I decide where I am flying and approximately when, I look at the options and identify the ones that don't have too tight or crazy-long connections and do not involve commuter airlines (I refuse to take those). I then check the fares on those acceptable flights nearly daily. Sometimes I look more than once a day. It takes very little time to do that on Google Flights. When I spot a significant difference in fares, I write down the date and the new fare. By doing that over a period of time (it's usually one or two months, but it could be longer as uncertain as things are now), I get a sense of what the typical fare is and can pounce if there's a sudden, worthwhile drop. Equally important, I will spot big upswings that most likely will not stick and avoid buying while the fare remains high.
One pattern that seems to hold true is that as you get closer to the date of travel, the gap between the bad routings (the ones taking way long because of very lengthy layovers, etc.) and the good routings (non-stop or with sane connections and layover times) expands significantly. If you care a lot about avoiding an unnecessarily long flight and refuse to spend lots of extra hours en route, you are well advised to buy your ticket a bit farther ahead. (No, I cannot tell you precisely what "farther ahead" means.)
Unfortunately, the travel situation is really messed up right now, with flights being added and many flights being canceled or rescheduled. The great deal you find and pounce on now may not actually exist when the time comes (so keep checking your ticket locator online to be sure there hasn't been a change the airline has not notified you about). Or you could settle now for the best of the rather unattractive options and then find that two or three months from now there are better flights on the schedule. It's a difficult time for all of us.